Dwyer first introduced spinal instrumentation for anterior spinal fusion in 1969. The Dwyer system used screws and a flexible cable. However, this system merely provided limited stability by the compressive effect of the one vertebral body against the other. The flexible cable resisted only tension forces, and the inability of the Dwyer system to provide a rigid connection between vertebrae often led to cable or screw failure with subsequent pseudarthrosis. In 1976 Zielke modified the Dwyer system by substituting a small diameter threaded rod and nuts for the cable and introduced a derotator designed to correct rotation and to prevent kyphosis. Over time, the Zielke instrumentation procedure for management of thoracolumbar and lumbar scoliosis was recognized to have significant advantages over the Dwyer system in terms of effective correctability of the coronal curvature, the ability to correct deformity by instrumenting shorter segments of the spine and also its derotation capability. However, the Zielke instrumentation has been reported to have high incidence of implant breakage, loss of correction, progression of the kyphosis, and pseudarthrosis caused primarily by lack of segmental stiffness in the relatively small diameter rod.
With the introduction of a larger diameter solid rod system by TSRH in 1989, creation of lordosis in the instrumented segment was possible by the appropriate contouring and rotation of a larger diameter rod. The 300% to 400% increased stiffness of the 6.4 mm rod over that of the Dwyer or Zielke longitudinal members was expected to provide stiffness sufficient to increase fusion rates while maintaining correction without external immobilization. However, after review of cases performed with the instrumentation, the incidence of loss of correction in both frontal and sagittal planes remained unacceptable, though improved compared to Dwyer and Zielke.
Some authors documented significantly high strains at the bone-screw interface of the cephalad and caudal end vertebral screws. Loss of correction and kyphosis in the single rod instrumented segment probably resulted from insufficient construct stiffness in the early postoperative period as a consequence of bone-screw interface loosening, especially at the cephalad and caudal interspaces. The single rigid rod may provide sufficient stability for the correction of the deformed spine during the early postoperative period. However, it may not prevent the vertebral rotation about each screw axis at the bone-screw interface during everyday activity. The possible reason is that the single solid rod system lacks two fixation points on each vertebra, particularly in the most cephalad and caudal end vertebrae of the instrumented segment.
In 1996, Kaneda introduced a two-rod anterior system (KASS) with two fixation points on each vertebra for management of thoracolumbar and lumbar scoliosis. The KASS system seemed to address some of the problems associated with prior systems, preventing the end vertebrae from rotating into kyphosis. This technique has been performed with good results in the early follow-up period. However, this system has some limitations; namely, the system has a high prominent profile and is difficult to apply to a severely deformed spine.
To combine the relative ease of implanting a single rigid rod structure with two fixation points on each end vertebra, the rod-plate anterior system of the present invention is an improvement over the single solid rod anterior system (TSRH), and also allows spinal plate fixation at the cephalad and caudal end vertebrae of the instrumentation segment.